1. Field of the Invention
The present invention relates to a vehicle behavior control apparatus which applies braking force to wheels of the vehicle on an inside of a turning circle in accordance with a difference between the standard yaw rate and the yaw rate detection value at the time of vehicle turning, and more particularly to a technique for suppressing an undesirable vehicle behavior as occurring at the time of failure of the yaw rate detection unit.
2. Description of Related Art
In the event that a vehicle slips sideways or skids in turning due to understeer or oversteer, the vehicle deviates from a target traveling course intended by the driver of the vehicle. To cope with this, a control has been adopted to cause the vehicle to return to its target traveling course by apply brakes on either left wheels or right wheels when it is judged that the vehicle has deviated from the target traveling course.
In this case, the deviation from the target traveling course is normally judged by comparing a standard yaw rate, which is calculated from a vehicle velocity obtained by a vehicle velocity detection unit and a steering angle obtained by a steering angle detection unit, with a yaw rate detection value detected by a yaw rate detection unit.
For example, when a value resulting from subtraction of the yaw rate detection value from the standard yaw rate exceeds an understeer judgment threshold, it is judged that the vehicle is in an understeer state, and then, braking force is imparted to wheels of the vehicle on an internal side of a turning circle. On the contrary, when a value resulting from subtraction of the yaw rate detection value from the standard yaw rate is less than the understeer judgment threshold, it is judged that the vehicle is in an oversteer state, and then, braking force is imparted to wheels of the vehicle on an external side of the turning circle (refer to Japanese Patent Unexamined Publications JP-A-7-117645 and JP-A-2004-284485).
In vehicle behavior control apparatuses disclosed in the JP-A-7-117645 and JP-A-2004-284485, when a yaw rate detection unit or an electronic component within an ECU fails (operation failure or contact failure) while the vehicle is turning, resulting in a yaw rate detection value detected by the yaw rate detection unit becoming too small, it is undesirably judged that the vehicle is in an understeer state. Then, there is a possibility that a relatively strong braking force is imparted to the wheels of the vehicle on an internal side of a turning circle. As this occurs, the vehicle behaves in such a manner as to turn towards the internal side of the turning circle (namely, the vehicle is put in an oversteer state) relative to its target traveling course. Thus, there is a possibility that the driver is forced to perform a countersteer operation or feels an unstable steering feel.